refactor(ast): unify AstVisitorBase Stmt.Expression handler name (#1250)

[nickna]

Summary

Renames VisitExpressionStmt → VisitExpression in the AstVisitorBase analyzer-visitor family, so the handler for Stmt.Expression follows the same mechanical Visit + node-type-name rule used everywhere else (Stmt.Var → VisitVar, Expr.Binary → VisitBinary, …).

Stmt.Expression was the only node whose handler name diverged between the two dispatch families:

Family	Handler for Stmt.Expression (before)
Analyzer visitors (AstVisitorBase + subclasses)	VisitExpressionStmt
Registry dispatch (Interpreter, TypeChecker, via NodeRegistry.AutoRegister's $"Visit{Name}")	VisitExpression

This removes the last exception to the uniform naming and the one wrinkle a future source-generator-based dispatch (the road not taken in #1243) would have to special-case.

Changes (4 spots, behavior-neutral)

• Parsing/Visitors/AstVisitorBase.cs — the Stmt.Expression switch case
• Parsing/Visitors/AstVisitorBase.cs — the virtual method definition
• Compilation/StringAccumulatorPromotionAnalyzer.cs — the override
• Compilation/StringAccumulatorPromotionAnalyzer.cs — the base.VisitExpression(stmt) call

No collision with the existing Interpreter.VisitExpression / TypeChecker.VisitExpression methods — those live on the registry-dispatch types, not in the AstVisitorBase hierarchy.

Testing

• dotnet build — 0 errors
• Full xUnit suite — 15415 passed / 0 failed (includes AstDispatchTests, which guards the AstVisitorBase switch via a GetUninitializedObject + NotSupportedException probe)

Pure internal C# method rename with no behavioral surface — the Test262/TSConf conformance suites exercise TypeScript semantics this change cannot affect.

Part of #1094.

Closes #1250